Global issueMarine litter is any solid material that is discarded or lost on beaches and shores or at sea, including materials transported into the marine environment from land by rivers, draining or sewage systems, or wind. Litter originates primarily from land-based sources (such as littering, industrial facilities, and extreme natural events) and to a lesser extent oceanic sources (such as fishing vessels, cargo ships, and aquaculture installations). It is found in all the world's oceans, not only in densely populated regions, but also in remote areas far from obvious sources and human contact. Huge amounts of plastics, metals, rubber, paper, textiles, derelict fishing gear, vessels, and other items enter the water every day, making marine litter one of the most widespread pollution problems.

Litter takes on many forms, in terms of size as well as type of material. Plastics are particularly widespread in the seas and oceans. Their durability and low costs allow for their successful use in industries and everyday life, but this also means they persist in the marine environment, where they cause harm to a great variety of organisms, including humans.

This litter causes environmental, economic, health, and aesthetic problems and is a challenge with significant implications for the environment as well as for humans. Impacts are cultural and multi-sectoral, resulting from various human activities, poor waste management, a lack of infrastructure, insufficient consideration of plastic use, the absence of satisfactory legal and enforcement systems, and a lack of financial resources.

Damaging impactsLitter has a serious impact on marine organisms and leads to the degradation of marine habitats. Animals can easily get entangled in ghost-netting, rope, and monofilament fishing lines. Plastic is often ingested, clogging intestinal tracts, leading to starvation, death, or tissue infection, ultimately reducing life quality and reproductive performance. Debris like lost or abandoned fishing gear can drag across the seabed, damaging benthic habitats or endangering reef structures. Flotsam can also transport harmful, invasive species. Some plastics, such as microbeads, arrive in very small sizes, together with fragments from microplastics (below 5mm). Microplastics are ingested by many marine organisms and move up the food chain; in this way, humans can also end up consuming contaminated organisms, placing their health at risk.

Coordinating effortsThe persistence of marine litter results from a lack of strategy and an absence in the implementation and enforcement of existing programmes, regulations, and standards at all levels. Recognizing the importance of cross-sectorial and international cooperation, several regional and global organizations and initiatives, such as OSPAR Regional Action Plan and the G7/G20 Marine Litter Action Plan, have been launched. These focus on addressing pollution, putting in place effective actions, and financial aspects such as new sources of funding for effective waste management as well as addressing education, outreach, and research requirements.

In 2016, the UN Sustainable Development Goals (SDGs) officially came into force. For SDG14, countries made commitments to reduce marine litter, including as part of the UN Environment's 'Clean Seas Campaign.' In Europe, marine litter not causing harm is the subject of Descriptor 10 of the Marine Strategy Framework Directive (MSFD), through which good environmental status (GES) must be achieved by 2020. Despite all this, good practices for adequate monitoring or impact determination are relatively sparse.

Monitoring methodsApproaches to detecting and estimating litter can either be targeted or opportunistic. Monitoring costs can be high due to the involvement of a vessel or the need to monitor large water or beach areas. Such monitoring is expected to mostly be connected to other activities. Though this can drastically reduce ship costs, monitoring needs dedicated protocols and personnel. The work can be done by volunteers on beaches or personnel with different occupations on a ship. This requires careful planning, as the requirements for several tasks on board may not be compatible. Direct detection from ships remains primarily traditional visual observation (measuring, photographing, and noting all objects). As with all such systems, vessel and weather conditions affect detection.

Certain European programmes for monitoring seabed litter have started through opportunistic sampling. Programmes for demersal fish stocks, undertaken as part of the International Bottom Trawl Surveys (IBTS), operate at large regional scales and provide data using a harmonized protocol. This provides consistent support for monitoring seabed litter at the European scale and within MSFD requirements. For deep-sea areas, the use of submersibles or remotely operated vehicles could form a possible approach.

Across parts of the OSPAR region, floating litter is monitored by analysing the stomachs of dead fulmars, which are gathered by birdwatchers. Citizen science and beach cleans have proven invaluable in the monitoring efforts on beaches. Given the potential to collect and process much larger datasets than could be done by traditional science alone, citizen science can potentially have an important role in litter monitoring in the future – as part of a solution alongside traditional monitoring, remote sensing, and modeling.

Looking aheadThere are many possibilities for improving detection, coverage and accuracy, at sea. Computer models can be used to simulate movement of litter or to estimate where it has been or is going. Several digital camera systems and subsequent analysis by image recognition techniques have been proposed recently. The opportunistic use of (un)manned aerial surveys, satellites, and image collection systems on vessels for covering a large area rapidly has been considered. This has been useful to identify large litter patches, providing useful information for targeted clean up actions and model calibrations.

Future efforts should focus on the development of (automated) systems that collect marine litter data from existing platforms such as ships, airplanes, and satellites. This could be paired with image recognition to quantify the items and keep processing costs down. Similar automated techniques should be developed to detect microplastics. Finding ways to partner and integrate marine litter monitoring with existing surveys is crucial, although there is a need to standardize and expand opportunistic reporting.